1. Field of the Invention
The invention relates to a voltage-regulating device having first and second power supply terminals and an output terminal at which the device supplies, under a variable output current, a regulated output voltage, the device including a buffer circuit which comprises:
first and second transistors arranged in series between the first power supply terminal and the output terminal, PA1 a comparison module having one output and two inputs, one input receiving at least a portion of the output voltage and the other receiving a reference voltage, PA1 a module for controlling the distribution of the energy dissipated by the device, driven by the output of the comparison module. PA1 a voltage-divider bridge arranged between the output terminal and one of the power supply terminals, PA1 a comparator having one output and a first and a second input, the first input receiving the reference voltage, and the second input being connected to a junction point of the divider bridge, PA1 a fifth transistor whose base is connected to the output of the comparator, the emitter being connected to the second power supply terminal and the collector being connected to the base of the second transistor.
2. Description of the Related Art
A buffer circuit as described above is known from the article "Verminderen van warmtedissipatie" by W. Birkhoff, published in the Radio Bulletin of September 1977. The buffer circuit described in this article comprises a module for controlling the distribution of the energy, allowing the dissipation to be distributed substantially evenly among the two transistors. However, such a buffer circuit presents a problem when one wishes to realize it in the form of an integrated circuit. Since an integrated circuit includes components of reduced dimensions, it can dissipate only a low power, with the risk of destruction of these components. At a constant output voltage, the power which the buffer circuit must dissipate is proportional to the output current. The output current of such a buffer circuit is thus necessarily limited.
It is an object of the invention to remedy this drawback by providing a voltage-regulating device in which the power to be dissipated by the buffer circuit is reduced without the output current value being limited.
To this end, a voltage-regulating device according to the invention is characterized in that the device is provided with an electrical circuit allowing a part of the power dissipated by the output current to be discharged outside of the buffer circuit.
An embodiment of the invention provides a voltage-regulating device as defined in the opening paragraph, which is characterized in that the device comprises a first resistor, a first terminal of which is connected to the output terminal of the device and a second terminal is connected to the junction point between the first and second transistors, and in that the control module is provided with means for shunting at least a part of the output current in said first resistor.
A considerable part of the power is thus dissipated in the resistor and thus at the exterior of the buffer circuit.
An advantageous embodiment of the invention provides a voltage-regulating device as described above, which is characterized in that the control module comprises a third transistor arranged in a current-mirror configuration with the first transistor, the collector of the third transistor being connected to a terminal of a second resistor, the other terminal of which is connected to the base of the second transistor and to the output of the comparison module.
In such a device, the voltage drop at the terminals of the second resistor controls the conduction of the second transistor. In a first period, this transistor is not turned on and the quasi-total amount of power is dissipated in the first resistor outside of the buffer circuit. It is not until the output current becomes sufficiently large to saturate the first transistor that the second transistor is turned on, thus allowing the buffer circuit to internally dissipate a surplus of power which the first resistor cannot handle.
A variant of the invention provides a voltage-regulating device as described above, which is characterized in that the device comprises a fourth transistor arranged in a current-mirror configuration with the first transistor, the fourth transistor constituting a current source for biasing the comparison module.
This variant allows the buffer circuit to adapt its energy consumption to the conditions in which it operates. The currents which it consumes are substantially proportional to the current which flows through the first transistor, which current is equal to the output current. When this current is low, the current consumed by the overall buffer circuit is low.
A preferred embodiment of the invention provides a voltage-regulating device as described above, which is characterized in that the first and second transistors have a surface which is several times larger than that of the third and fourth transistors.
In such a device, the currents consumed by the third and fourth transistors are reduced to a fraction of the output current, which allows a considerable reduction of the power consumed by the buffer circuit.
A particular embodiment provides a voltage-regulating device as described above, which is characterized in that the first, second, third and fourth transistors are PNP transistors.
Another variant of the invention provides a voltage-regulating device as described above, which is characterized in that the comparison module comprises: